In sheet-processing printing machines and, in particular, sheet-fed offset printing machines, ink is supplied to the form or plate cylinder from an ink fountain roller, cooperating with ink-metering elements, via an intermittent vibrating roller and a number of inking rollers. The ink layer on the plate cylinder is additionally split via a rubber blanket and, thus, passes indirectly onto the print carrier. Disadvantageously, the large number of rollers and the accompanying splitting operations tend to slow the overall ink transfer process. Furthermore, a specific number of machine revolutions is required before a desired ink layer-thickness is established on the rollers of the printing unit. Correctly printed sheets are obtained only when an ink-layer thickness profile corresponding to the ink requirement of the print subject is established on the inking rollers. Therefore, if this ink-layer thickness is disturbed, for example, by a stoppage of the transportation of ink while the printing machine continues to run, the desired print results will not be achieved until the desired ink-layer thickness is again reestablished.
To address this problem, various methods have been developed to achieve as quickly as possible the desired ink layer-thickness gradient. For example, one known method employs a pre-fill of the inking unit with a specific ink quantity so as to give the inking unit specific transverse profiles after ink metering begins. Nevertheless, these methods serve only to shorten the setting-up operation from one print order to the next and fail to address the situation arising from an ink supply stoppage when the ink-layer thickness gradient established on the individual inking rollers during production is effectively destroyed.
In the case of an ink supply stoppage, once printing is resumed, with clearance for the movement of the vibrating roller and with the ink applicator rollers and the cylinders being switched on in the correct sequence to print, the first sheet and a number of those following will exhibit inking which deviates markedly from the desired inking. Such systems are described in, for example, U.S. Pat. No. 5,845,576 (corresponding to DE 196 13 360) and U.S. Pat. No. 5,533,448 (corresponding to DE 43 33 071). Generally, this deviation results from the fact that the leveling operations (i.e., splitting operations) proceed as the machine continues to run even when ink flow has stopped. Therefore, using the aforementioned method does not afford any improvement in this scenario since the resumption of the correct ink-layer thickness requires a multiplicity of machine revolutions without any paper runs thereby interrupting and further delaying the printing process. While it has also been proposed to carry out so-called "inking-unit separation" at the time of such an ink stoppage, i.e., to stop the roller train of the inking unit at a multiplicity of points thus causing the ink-layer thickness on the individual inking rollers to be frozen, this measure is nevertheless unsatisfactory since it is highly complicated in terms of construction and, moreover, the turning on and off of the inking rollers results in additional faults.